The present invention relates to a device for sensing the surface potential of a conductor, insulator or similar object with a sense electrode not contacting the object. More particularly, the present invention is concerned with a surface potential sensing device suitable for the control of the charge potential of, e.g., a photoconductive element or a developing roller included in a copier, facsimile apparatus, printer, plotter or similar image forming apparatus.
In various technological fields, it is often necessary to sense or measure the surface potential of a conductor, insulator or similar object. In a copier, for example, the surface potential of a photoconductive element must be accurately sensed and controlled in order to enhance image quality. It has been customary with a copier to use a noncontact type surface potential sensor capable of obviating the leak of charge from the object. The noncontact type sensor operates either electrically or mechanically. A problem with the electric sensor is that it is expensive due to the use of a special functional material. Another problem is that the sensitivity of the electric sensor falls due to contamination ascribable to deposits and the polarization of an insulator used. The mechanical sensor is predominant over the electrical sensor because it is free from the fall of sensitivity ascribable to contamination and comparatively inexpensive.
The mechanical sensor is provided with either a chopper type configuration or a vibrating-reed type configuration In the chopper type configuration, an electric line of force input to a sense electrode is interrupted periodically in order to vary the amount of charge to be induced on the electrode, thereby outputting an AC signal. In the vibrating-reed type configuration, a sense electrode is displaced periodically in the direction of an electric field extending from an object. As a result, electrostatic capacitance between the object and the sensor electrode varies periodically, allowing a varying AC signal to be output. Japanese Patent Publication No. 63-1547, for example, teaches a relatively simple and inexpensive surface potential sensor including a tuning fork serving a s chopper means, The tuning fork is caused to vibrate to output an AC signal representative of the surface potential of an object On the other hand, Japanese Patent Laid-Open Publication No. 60-120267 discloses a surface potential sensor in which a tuning fork or vibrator is caused to vibrate in a direction substantially parallel to an electric field formed between a sense electrode and an object. In this condition, an AC signal representative of the surface potential of the object is output with an enhanced SIN (Signal-to-Noise) ratio.
The conventional sensors described above have some problems left unsolved, as follows, When it is desired to sense the surface potential with a higher S/N ratio or sensitivity, the amplitude of vibration of the tuning fork or vibrator must be further increased. To increase the amplitude of vibration, the mechanical resonance vibration of the fork may be used, or the dimension of the fork in its lengthwise direction may be increased. However, the problem with the mechanical resonance vibration scheme is that the resonance point (resonance frequency) and therefore a surface potential signal representative of the amplitude of vibration is extremely unstable. This makes it difficult to detect an accurate surface potential. The problem with the elongate fork scheme is that a sensor probe becomes bulky.
The tuning fork is caused to oscillate by a piezoelectric material or an electromagnetic coil mounted thereon.
However, the piezoelectric material is susceptible to mechanical stresses, i.e., it is apt to crack or bread Further, when an overvoltage is applied to the piezoelectric material, the polarization of the material is destroyed and prevents the sense electrode from being displaced. Moreover, the piezoelectric material is relatively expensive. In addition, the width over which the material is displacable is susceptible to the environment (particularly temperature), so that the sensed values is apt to be unstable. On the other hand, the electromagnetic coil has a complicated structure. At the present stage of development, it is extremely difficult to obtain an electromagnetic coil as small as 1 cm.sup.3 or less on the market. This obstructs the miniaturization of the sensor probe.
In the conventional sensor arrangements, electrostatic capacity between the object and the sense electrode is proportional to the reciprocal of the distance between them, so that the output value varies noticeably along with the above distance. For example, assume that the sensor senses the surface potential of a photoconductive drum included in a copier and rotatable in a preselected direction. Then, whether a change in the output value is ascribable to the potential distribution of the drum or whether it is ascribable to the distance between the drum and the sense electrode cannot be determined. This makes it difficult to measure the surface potential with accuracy.
In light of the above, a distance correction type surface potential sensor allowing a minimum of change due to a change in distance to occur has been proposed in various forms in the past For example, U.S. Pat. No. 3,852,667 proposes a surface potential sensor including an integration type high-voltage generator. When an output signal is fed to the high-voltage generator, it generates a high voltage matching the input signal and feeds it back to the housing of a sensor probe. As a result, the potential of the object and that of the housing become equal and cancel electrostatic capacity existing therebetween. This prevents the output value from depending on the distance. However, the high-voltage generator brings about the following problems. The high potential which the sensor can sense is limited by the voltage generating ability of the high-voltage generator. The cost of the high-voltage generator increases with an increase in voltage. This increases the cost of high-voltage parts constituting a measuring circuit together with the high-voltage generator, and thereby increases the overall cost of the sensor. Further, a feedback circuit included in the device sophisticates the construction. In addition, the sensor must be handled with greatest care because a high voltage is applied to the sensor probe.
Japanese Patent Publication No. 3-20700 (Japanese Patent No. 1659026) and Japanese Patent Laid-Open Publication Nos. 62-118267 and 62-113072 each discloses a chopper type or a vibrating-reed type surface potential sensor. The chopper type or the vibrating-reed type sensor has two sense electrodes each being spaced a particular distance from an object. Two output signals of the sense electrodes are used to correct the fluctuation of the output ascribable to that of the distance between the object and the electrodes. However, the chopper type sensor seeds a chopper electrode for interrupting the electric line of force input to the sense electrodes, resulting in a complicated structure. Further, two or more sense electrodes are indispensable which further complicate the structure and renders the entire sensor bulky. Also, the increase in the number of parts increases the cost. In addition, because the sense electrodes are held stationary, dust sequentially accumulates on the electrodes due to, e.g., electrostatic adhesion. The dust lowers the sensitivity of the sense electrodes and obstructs accurate measurement. This is particularly critical when the sensor is built in an electrophotographic copier in which various kinds of dust including toner appear.
Japanese Patent Publication No. 4-45109 (Japanese Patent No. 1759275) and Japanese Patent Laid-Open Publication No. 6-308179 each teaches a vibrating-reed type or a chopper type surface potential sensor including a single sense electrode. In this type of sensor, two output signals different in phase or in timing are produced from the sense electrode and used to correct the fluctuation of the output ascribable to that of the distance between the object and the sense electrode. Specifically, in Publication No. 4-45109, the output is corrected on the basis of a difference between two phase outputs. However, when the sense electrode is caused to vibrate by a piezoelectric tuning fork, the level of a signal representative of the above difference is extremely low because the amplitude of vibration is limited Therefore, it is difficult to execute distance correction with a sufficient S/N ratio or to output a sufficiently accurate surface potential. The sensor of Laid-Open Publication No. 6-308179 has the problems described previously in relation to a chopper type sensor, Further, a sample and hold circuit included in this sensor is apt to scatter the measured values and obstruct accurate measurement. In addition, if the surface potential of the object varies while two output signals are produced, then an accurate surface potential is not achievable. A complicated detection circuit is needed in order to produce two output signals at different timings.